1) Field of the Invention
The present invention relates to composite sandwich panels employing honeycomb cores, and in particular relates to such composite panels designed to be used in environments requiring fire or flame resistance, or fire protection.
2) State of the Prior Art
Standard low FST (Flame, Smoke and Toxicity) composite panel structures are normally made of phenolic prepreg skins on Nomex.RTM. honeycomb core. An example of this can be found in Applicant's own U.S. Pat. No. 4,956,217. This patent provides a laminate structure that comprises a honeycomb core structure with facing sheets bonded thereto, the core being treated with a silica impregnant/coating to impart improved flame resistance and to reduce heat transmission and toxic emissions upon exposure of the core to direct or indirect heat, flame and/or a pirolytic environment. The use of the honeycomb core combined with rigid skin panels forms a sandwich structure providing semi-structural strength. In addition, this patent provides a honeycomb composite structure that exhibits excellent heat and flame barrier properties, while still retaining adequate mechanical strength.
U.S. Pat. No. 4,251,579 to Lee et al. discloses an alternate fire protection panel, wherein the cells of a honeycomb core are filled with a fire-extinguishing fluid. Thus, any projectiles impacting and passing through the facing sheets would cause the fire-extinguishing fluid to be rapidly liberated to suppress or extinguish any fire caused by entry of the projectile.
U.S. Pat. No. 5,106,668 to Turner et al. provides a multi-layer honeycomb core structure having a plurality of core layers of different densities and thicknesses. The emphasis in this patent is the maintenance of a low mass density while providing good strength in resistance to compressive bending stresses and high resistance to damage from drops or other impacts, as well as resistance to water ingression.
U.S. Pat. No. 4,937,125 to Sanmartin et al. provides another multi-layer core sandwich structure that is resistant to both impact and thermal aggressions. A synthetic cellular material having a low thermal conductivity coefficient provides one layer, while another layer is made up of an off-the-shelf extruded polystyrene polyvinylchloride, or polyethene plate.
However, none of the above provides a composite sandwich panel that provides a 15 minute burn-through protection at 10.5 btu heat flux density at 2000.degree. F.